1. Field of the Invention
This invention relates to iris imaging systems and to deformable minors, for example as may be used in adaptive optics systems.
2. Background
Biometric identification via iris imaging relies on capturing images of a person's iris and then matching the captured image against a database of previously acquired iris information. In order for this approach to be effective, the subject's iris image must be captured with a certain amount of resolution. Low resolution images do not provide enough information to uniquely match against the database. Furthermore, in many applications, it is desirable to capture iris images from many subjects quickly, non-invasively, and from a distance.
However, many iris imaging systems require correction of optical aberrations (including, for example, focus correction) that would otherwise negatively affect the accuracy, reliability and/or efficiency of the system. To address the problem of optical aberrations in various applications (including iris imaging), adaptive optics systems have been developed to compensate for aberrations. Adaptive optics have been suggested for use in free-space optical communications (e.g., to compensate for atmospheric aberrations and/or to provide fast tracking and focus), astronomy (e.g., to compensate for atmospheric aberrations suffered by land-based telescopes) and, more recently, for iris imaging systems (e.g., to provide fast tracking and auto-focus control).
Some adaptive optics systems use a deformable mirror as the “adaptive” element. However, different types of adaptive optics systems may require differing functionalities and/or optimizations of a deformable mirror. Thus, a deformable mirror that is optimized for one application may not be suitable for a second different application. Conventional designs for deformable mirrors have a number of deficiencies.
One type of deformable minor used is a stack actuator mirror. A stack actuator minor includes a number of push rods that engage the back of a flexible minor so that the extension and retraction of each push rod causes an associated deformation in the minor. However, this type of deformable minor can be expensive, large and relatively slow, since it requires the construction, assembly and alignment of a number of individual push rods. In addition, these deformable mirrors can also exhibit a waffle pattern on the mirror surface, and even higher order modes due to print through. When a small number of actuators are used regular geometry of such a mirror may provide a poor match to the required correction modes for the optical system. Moreover, the number of push rods, the closeness of the push rods, and the length of their travel are physically limited. Since all actuators have the same travel and are attached to a rigid reference surface, the minor has the same stroke for all modes (i.e., low order focus has the same stroke as the highest mode produced by every other actuator being turned on and off.) For correcting the aberrations originating in the atmosphere or many other aberration sources, this range of stroke at the highest modes is not necessary, but the stroke may be inadequate for low order modes. Accordingly, the accuracy and degree of optical correction that can be applied by the stack actuator type minor is also limited in many situations.
Another type of deformable mirror that is becoming commonly used is based on micro-machined-structures (MEMS) fabrication techniques. These mirrors typically have a large number of actuators, but the stroke of the deformable mirror is limited, and they often exhibit high angle scatter due to small scale manufacturing defects. The promise of MEMS technology is low unit price due to economies of scale, but the technology has not yet advanced to the point where low price can be achieved.
Thus, there is a need for improved deformable minors, for use in adaptive optics and other systems.